This invention relates generally to communications and, more particularly, to high-speed data communications systems.
Plain Old Telephone Service (POTS) is typically deployed to individual subscribers over a twisted pair of wires. Today, in addition to voice services, more and more subscribers want high-speed data access to, e.g., the Internet, over this twisted pair. A technology that increases the transmission capacity over a twisted pair is Asymmetric Digital Subscriber Line (ADSL), which provides transmission capabilities of millions of bits per second (Mbps),
One standard for ADSL transmission is ANSI T1.413 (e.g., see xe2x80x9cStandards Project for Interfaces Relating to Carrier to Custom Connection of Asymmetrical Digital Subscriber Line (ADSL) Equipmentxe2x80x9d, T1E1 Specification, 1997). This standard specifies the use of Discrete Multi-Tone (DMT) modulation. DMT modulation is a form of orthogonal frequency division multiplexing (OFDM) and utilizes multiple carriers (also sometimes referred to as subcarriers, channels, or bins; hereafter the term bin is used) for conveying information. In DMT modulation, the allocated frequency range is divided into K bins, K greater than 1, each bin separated by approximately 4 kHz. In such an approach, a DMT-based ADSL system transmits what is referred to as xe2x80x9cmulti-tone symbolsxe2x80x9d or xe2x80x9cDMT symbols.xe2x80x9d
In discrete multitone (DMT) receivers, a frequency domain equalizer (FEQ) is used to converge the symbol constellation for each bin. The FEQ is normally a one-tap complex finite impulse response (FIR) filter adapted by a Least Mean-Square (LMS) based algorithm (e.g., see, Chow, J. K., Tu, J. C. and Cioffi, J. M., xe2x80x9cA Discrete Multitone Transceiver System for HDSL Applications,xe2x80x9d IEEE J. on Selected Areas in Commun., vol. 9, no. Aug., 6, 1991; and Lee, I. Chow J. S. and Cioffi, J. M., xe2x80x9cPerformance Evaluation of a Fast Computation Algorithm for the DMT in High-Speed Subscriber Line,xe2x80x9d IEEE J on Selected Area in Commun. vol. 13, no. 9, 1995). Unfortunately, the convergence rate is slow (e.g., on the order of 20 secs.) if the FEQ is updated by the LMS algorithm or most other standard equalization algorithms. This slow convergence is a result of the dependency of convergence rate on received signal power. For example, for an 8 kft (kilo-feet) loop, received signals (bins) at high frequencies have about 45 dB more power attenuation than the received bins at low frequenciesxe2x80x94resulting in different convergence rates for different parts of the received DMT signal.
We have realized a method and apparatus that improves convergence speed and enhances tracking capability in a DMT receiver. In particular, and in accordance with the principles of the invention, a DMT receiver uses an automatic gain control (AGC) element and a rotator element for use in equalizing the received DMT signal in each bin. As a result, the dependency of convergence rate on input power is reduced.
In an embodiment of the invention, during start-up a DMT receiver uses an automatic gain control (AGC) element and a rotator element in place of a one-tap complex finite impulse response (FIR) filter (as noted above updated using the LMS algorithm). This technique is applicable to receivers using non-blind equalization and blind equalization. In the latter case there is an additional benefitxe2x80x94it eliminates the need for a training sequence.